Moving-Iron Instrument

Moving-Iron Instrument


a measuring instrument whose operation is based on the interaction between a magnetic field that is proportional to the quantity to be measured and a core made of a ferromagnetic material. The main elements of a moving-iron instrument are a measuring circuit that converts the quantity to be measured into a DC or AC current and a measuring mechanism consisting of a moving-iron system (Figure 1).

An electric current in a field coil generates an electromagnetic field that pulls the core into the coil, thereby exerting, on a shaft, a torque that is proportional to the square of the current flowing through the coil. As a result of the action of a spring on the shaft, a countertorque is developed that is proportional to the angle of rotation of the shaft. When the torque and the countertorque interact, the shaft and an attached pointer are rotated through an angle that is proportional to the square of the quantity to be measured. When the torque and the countertorque are equal, the pointer comes to rest.

Figure 1. A moving-iron instrument: (1) coil, (2) core, (3) shaft, (4) pointer, (5) scale, (6) spring; (l) current

Moving-iron ammeters and voltmeters are manufactured, mainly for measurements in 50-hertz AC circuits. In a moving-iron ammeter, the field coil is connected in series to the circuit containing the current to be measured; in a moving-iron voltmeter, it is connected in parallel. Moving-iron measuring mechanisms are also used in ratio meters. The most widely used moving-iron instruments are switchboard instruments of classes 1.5 and 2.5, although instruments of class 0.5 and even of class 0.1 are manufactured. The latter instruments may be used for frequencies of up to 800 hertz.


Elektricheskie izmereniia. Edited by E. G. Shramkov. Moscow, 1972.
Elektricheskie izmereniia, 14th ed. Edited by A. V. Fremke. Leningrad, 1973.